Single level per check

[reuvenharrison]

This PR splits checks that have multiple severity levels into multiple checks, each with a single level. This will allow us to:

• make severity levels customizable (see https://github.com/Tufin/oasdiff/discussions/572)
• correct oasdiff checks output
• continue making the checks model more structured which will improve their coverage and accuracy (see https://github.com/Tufin/oasdiff/issues/580)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.12903% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.92%. Comparing base (ea17e6d) to head (886cc2a). Report is 3 commits behind head on main.

Files	Patch %	Lines
checker/rules.go	92.59%	4 Missing :warning:
checker/config.go	87.50%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #579 +/- ## ========================================== + Coverage 85.89% 85.92% +0.02% ========================================== Files 228 227 -1 Lines 12974 13027 +53 ========================================== + Hits 11144 11193 +49 - Misses 1415 1420 +5 + Partials 415 414 -1 ``` | [Flag](https://app.codecov.io/gh/Tufin/oasdiff/pull/579/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tufin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/Tufin/oasdiff/pull/579/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tufin) | `85.92% <96.12%> (+0.02%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tufin#carryforward-flags-in-the-pull-request-comment) to find out more.

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